Changes in the composition of our blood as we age may cause the deterioration of memory and other brain functions by damaging connections in the brain. Filtering younger blood into an older body could combat the problem by rejuvinating old tissue and keeping nerve cells in good working order, according to a new study. It means people in their 40s or 50s could in future be given blood donated by someone in their early twenties to prevent their brain from deteriorating and stave off diseases like Alzheimer's.